Page 116 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

Therefore, recycled aggregates produced from 2. Eccentric-shaft rotor methods, in which
mobile crushers are usually used as fill or capping the recycled aggregates are passed between
layers, and used at, or close to, the location that two cylinders eccentrically rotating at high
they were crushed. In contrast, recycled aggregates speed which separates coarse aggregates
from central recycling facilities undergo a number from mortar via a grinding effect, and
of processes to ensure higher quality. This may 3. Straightforward mechanical grinding,
include: magnets, picking stations (although these in which the aggregates are placed in a
are becoming rare), trash screens, screens, log rotating drum containing iron balls.
washers, water pumps and sludge tanks.
Some initial trials at the University of Dundee have
shown that these do indeed improve recycled
Furthermore, central recycling facilities take great
care to control the type of CDEW that is allowed aggregate performance, but these techniques are
not yet practiced in full-scale in the UK.
to be stockpiled. In the UK, central recycling
39
facilities are known as washing plants and it is UK
practice to avoid crushing of CDEW if it can be at STANDARDS AND SPECIFICATIONS
all avoided. However, when it is necessary, a series There are few standards and specifications
of successive crushers (or crushes) and screening currently in place for use of recycled aggregates,
should be used. Impact crushers are beneficial as and where they are allowed it is standard practice
they give RA a more rounded shape which aids to subject recycled aggregates to the same rules as
engineering performance. natural aggregates. This tends to be an obstacle to
their greater use. However, standards have been
introduced in Europe by CEN (largely based on
Soil, silt and clay can be particular problems and the composition of the recycled aggregates) and
care is usually taken to avoid these materials in Japan (based on a combination of performance
entering the CDEW stream at source. On site, related properties and composition).
contractors use a scalping screen ahead of the
primary crusher to remove soil and clay balls; EUROPEAN (CEN) PRACTICE
whilst washing plants, are able to rely on the soil The opportunities for using domestic, industrial
and clay being removed during washing processes. and construction arisings in higher value
Furthermore, innovative methods, for example applications in the EU have expanded since 2002
the creation of filter cakes for use in drainage, are due to the introduction of the European standard
used to avoid the resulting soil, silt and clay being for aggregates for concrete, BS EN 12620, which
directed to landfill.
defines natural, manufactured and recycled
aggregates (Table 3), as: 5
In Japan, there have been attempts to further
improve the quality of recycled aggregate, via for 1. Natural aggregates, are from mineral
example the methods briefly stated below. 40
sources and subject to nothing more than
mechanical processing,
1. Heating and rubbing, whereby the recycled
aggregates are heated to 300ºC to remove
weaker mortar and cement particles from
the aggregate,

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